R. G. van Silfhout

The influence of Sb as a surfactant on the strain relaxation of Ge/Si(001)

We have investigated by the use of surface x‐ray diffraction the initial strain relaxation of Ge on Si(001) when a ‘‘surfactant’’ layer of Sb [0.7 monolayers (ML)] is present. Due to the high sensitivity of the technique to lateral strain in the overlayer, we have been able to observe directly the onset of strain relaxation at a coverage of 9–10 ML. This strain relief proceeds gradually as a function of coverage, but unlike the case without an Sb surfactant, it was not possible to relax the overlayer fully. No bulklike Ge was seen even up to a coverage of ≊55 ML. Concurrent specular reflectivity measurements also showed that the overlayer formed in a layer‐by‐layer mode up to the same level. These results differ quite markedly from those obtained without the surfactant layer, where several stages of island formation and strain relaxation are seen. They show dramatically how the modification of surface energies by the presence of a surfactant can affect the morphology of, and strain in, a lattice mismatched overlayer. In addition, they are direct confirmation that the initial strain relaxation mechanism in operation when a surfactant is not present is the formation of coherent Ge islands, rather than that of dislocations.

Asymmetrical dimers on the Ge(001)-2 × 1-Sb surface observed using X-ray diffraction

The atomic structure of the 2 × 1 reconstruction induced by the adsorption of Sb on Ge(001) has been determined by X-ray diffraction. Sb can be grown on Ge(001) in large ordered domains at elevated temperatures. SbSb dimers replace the Ge dimers of the clean Ge(001) surface and pick up all dangling bonds. The dimers have a bond length of 2.90 A and are midpoint-shifted by 0.16 A with respect to the substrate bulk unit cell. Such an asymmetric dimer is reported for the first time for a group IV/V system. Relaxations of the four topmost substrate layers are measured as well, and these compare favourably to elastic strain calculations.

Strain relaxation during the surfactant modified epitaxial growth of Ge/Si(001)

The initial strain relaxation of Ge on Si(001) has been investigated during epitaxial growth modified by a “surfactant” layer of Sb. Grazing-incidence X-ray diffraction was used to measure the strain relaxation due to its high sensitivity to the in-plane distribution of lattice spacings. We have observed the critical thickness for strain relaxation in the Ge overlayer to be ~ 11 monolayers (ML), with further relaxation developing gradually and in stages. A meta-stable, partially relaxed layer forms up to a coverage of ~ 30 ML, after which a more pronounced relaxation takes place. Even at a coverage of ~ 55 ML, complete strain relief has not been reached, and was only achieved after a 700°C thermal anneal. Concurrent specular reflectivity measurements also reveal that the Ge overlayer grows in a layer-by-layer fashion, and not in the Stranski-Krastanow mode expected for the Ge/Si system. These data show dramatically how the modification of surface energies through the presence of a “surfactant” can affect the morphology of, and strain in, a lattice-mismatched heteroepitaxial system.

An X-ray diffraction study of the Si(111) (√3 × √3) R30°-indium reconstruction

Abstract The structure of the (√3 × √3)R30° reconstruction induced by the adsorption of 1 3 of a monolayer of In on the Si(111) surface has been determined using surface X-ray diffraction. The fractional order Patterson function obtained from structure factor intensities at zero perpendicular momentum transfer (l = 0), indicates lateral displacement in the silicon surface atoms. Intensity profiles of fractional order rods and one integer order rod gives information concerning displacements normal to the surface of the silicon substrate. The indium adatoms are shown to occupy the 4-fold coordinated T4 sites above the second layer silicon atoms. Keating elastic strain energy minimisation has been used to determine relaxations down to the sixth layer of the bulk.

The initial stages of growth of Ge on Si(001) studied by X-ray diffraction

The structure of an ultra-thin epitaxial Ge layer during in situ deposition onto a Si(001) surface has been investigated. Peaks arising from the 2*1 reconstruction disappear at a coverage of approximately=ML. The layer is epitaxial with respect to the substrate up to a coverage of approximately=5 Ml, beyond which the strained layer relaxes gradually. At a coverage of 10 ML the lateral strain is reduced to 2.2% compared with 4.0% in the unrelaxed layer.

Ion and X-Ray Scattering Studies of Surface Disorder

We discuss two different types of surface disorder: atomic-scale roughness and disorder induced by surface melting. The roughening of a surface during, e.g., a crystal growth process can be monitored in situ by X-ray scattering. As an example the deposition of Ge atoms on Ge(111) is followed in time. For the detection of disorder induced by surface melting the technique of medium-energy ion scattering can be used to advantage. We will discuss recent ion scattering studies of the thermal disordering of the Al(110) and Ge(111) surfaces.

Strain Relaxation in Ge/Si(001) Studied Using X-Ray Diffraction

Grazing incidence x-ray diffraction has been utilized to give a direct measure of the lateral strain distribution during in-situ MBE deposition of Ge onto Si(001). Differences in growth conditions and thermal treatment result in significantly different strain relaxation behavior. The results demonstrate the gradual relaxation of strain, which is incomplete at a coverage of 11 monolayers. At this coverage the strain distribution exhibits two components, one of which is almost fully relaxed and the other having a range of lattice spacings intermediate between those for bulk Si and Ge. The results are discussed in terms of current models for strain relaxation.